1. Field of the Invention
This invention relates to a thermotropic liquid crystal polymer (referred to as LCP) composition and, more particularly, to an electrical insulator which, when injection molded, is suitable for use in the structure of electric machines and appliances.
2. Description of the Prior Art
It has been common practice to reinforce liquid crystal polymers with fillers but the filler loading has typically been up to 30-40 wt. % and it has not been known to improve the insulating and heat conducting properties of the polymers by loading them with fillers through injection molding.
Magnetic iron cores in the stators of small motors, stator coils mounted on the cores and their terminals are exposed to various adverse conditions during practical use such as mechanical stresses due to the pulsating torque and heat generation that accompany motor operation, as well as moisture, water, corrosive atmosphere, corrosive liquids and heat dissipation and, hence, studies have been made to develop effective methods for securing and insulating those members.
A common method adopted to this end comprises insulating stator coils with insulating varnish while securing them to a stator's magnetic iron core using an adhesive, then providing a housing using cast metal parts. However, this method involves not only many steps of manufacture but also much time and materials, thereby increasing the cost of the product. Furthermore, heat and cold cycles and mechanical vibrations that are produced during the operation of rotating machines have frequently caused debonding or deterioration in the insulating performance. These defects are particularly marked in motors that are caused to run at high speed at high frequency on a variable frequency power supply, as by PWM inverter control (e.g. variable voltage, variable frequency control, generally referred to as "VVVF control").
With a view to solving the problem, a method has been proposed that performs resin molding on the stator iron core and coils as a whole. The resin composition that is to be used in this method and which is capable of monolithic molding is required to meet the following requirements. First, the resin composition must exhibit very good adhesion to the stator's magnetic iron core and coil. Second, it must retain its insulating quality for a prolonged time while enhancing the dissipation of heat generated during the operation of a rotating machine and yet without causing debonding. Third, it is required that heat and cold cycles due to the start and stop of the rotor or the vibrations that occur during operation should not develop cracks in the molding layer to deteriorate the insulating performance or dimensional precision of the machine, thereby insuring that the rotor will turn smoothly. Fourthly, electromagnetic considerations for rotating machines require that the many slots in the stator's magnetic iron core should be completely filled with the resin to avoid any disturbances in the distribution of electric lines of force while, at the same time, the pulsating vibrations that occur during the operation of the rotating machine should not cause the resin to separate from the core, develop cracks or break into pieces that collide against the rotor.
Most importantly, the resin composition must have good insulating quality among other things. Once molding is accomplished, retouching is no longer possible and, hence, absolute reliability is required for the characteristics of the resin composition.
For resin molding of conventional electric machines and appliances, liquid unsaturated polyester resins, vinyl ester resins and epoxy resins have been used by means of cast molding, low-pressure molding or vacuum cast molding, but this method has caused the following problems in terms of manufacture and performance: (1) long curing time; (2) the filler is precipitated during curing to form a resin-rich portion and, hence, cracking is prone to occur on account of heat cycles: (3) the shaped part must be enclosed with a casing (container) and this makes it difficult to produce complexly shaped parts: (4) air voids are prone to occur in the shaped part but they are not desirable from the viewpoint of performance; and (5) the part being cured is highly susceptible to the effect of atmospheric moisture. In short, cracking will occur during resin molding by the prior art technique on account of various factors including shrinkage upon curing, heat cycles, air voids and the difference in the degree of curing, and this has made the occurrence of time-dependent deterioration unavoidable.
It has generally been held that solving all of the aforementioned problems of the prior art is extremely difficult. An injection molding technique that is capable of high filler loading and high production rate is useful as a means for solving the problems but injection molding materials so far available have defied injection molding since they are liquid resins which involve chemical reaction.